1. Field of the Invention
The present invention relates to an image forming device and a method for controlling a divisional light scanning device, and particularly relates to a method of controlling a divisional light scanning device for controlling a divisional light scanning device which scans respective light beams on an image carrier such that an exposure range on the image carrier is divided into plural partial exposure ranges in advance in accordance with plural light beams along a scanning direction of the light beams, and an optical scanning range of each light beam includes a corresponding partial exposure range and ranges from a boundary of this partial exposure range to a position spaced apart therefrom by a predetermined length along the aforementioned scanning direction. The present invention also particularly relates to an image forming device to which this method of controlling a divisional light scanning device can be applied.
2. Description of the Related Art
In general, in an image forming device such as a laser printer, a laser copying machine, or the like, a photosensitive body is charged by a charging means and a light beam is scanned on the photosensitive body by a light scanning device so that an electrostatic latent image is formed on the photosensitive body. A toner image is formed on the photosensitive body by developing the electrostatic latent image by a developing means. The toner image formed on the photosensitive body is transferred onto a transfer material so that an image is formed on the transfer material. Increasing the formable image size and increasing the image forming speed are always demanded of this type of image forming device. Accordingly, broadening of the scanning range (exposure range) of the light beam on the photosensitive body and increasing the scanning speed are required of the light scanning device.
However, when the scanning range of the light beam is widened in the light scanning device, a problem exists in that the optical member such as an f.sup..theta. lens or the like becomes larger so that the light scanning device itself becomes larger. Further, the scanning speed of the light beam can be increased by, for example, increasing the number of faces of a rotary polygon mirror for deflecting the light beam. However, when the number of faces of the rotary polygon mirror is increased the while the optical characteristics of the light scanning device are maintained, the rotary polygon mirror becomes larger, and thus, the load applied to a motor for rotating the rotary polygon mirror is increased. Therefore, problems such as windage loss, vibration, and the like are caused. The scanning speed of the light beam can also be increased by increasing the rotating speed of the rotary polygon mirror. However, a motor capable of rotating the rotary polygon mirror at high speed is expensive and a problem also exists in that vibrations are caused as the rotating speed is increased.
To solve the above problems, Japanese Patent Application Laid-Open (JP-A) No. 63-47718 proposes a divisional light scanning device in which the exposure range on the photosensitive body is divided into plural partial exposure ranges along a scanning direction of the light beam, and plural laser beams emitted from plural laser light sources are incident on a reflecting face of the rotary polygon mirror at different incident angles such that each of the laser beams scans only a corresponding partial exposure range on the photosensitive body, and a single image is formed by the plural laser beams by modulating each of the laser beams in accordance with a partial image to be formed in each partial exposure range. In accordance with this divisional light scanning device, the scanning range can be widened and the time required for one scan can be shortened, i.e., the scanning speed can be increased, without causing problems such as an increase in the size of the device, an increase in cost, generation of vibrations, or the like.
However, in the above-described divisional light scanning device, exposure is performed by separate light beams in units which are the partial exposure ranges. Therefore, at a joint of adjacent partial exposure ranges, irradiating positions of light beams for exposing both partial exposure ranges are often slightly offset from each other due to discrepancies in the assembly positions of the respective optical parts forming the scanning device, or the like. A double exposure portion irradiated by both of the light beams or a non-exposed portion not exposed by any light beam are continuously generated along the joint of the partial exposure ranges due to this offset in irradiating positions. Accordingly, a problem exists in that a striped pattern is formed at a portion corresponding to the joint of the partial exposure ranges on an image. Further, there is the fear that, due to this striped pattern, the image formed by using the above divisional light scanning device will be visually recognized as a collection of fragmentary partial images with the partial exposure range as a unit. Accordingly, there is the possibility that image quality will suffer greatly.
To solve this problem, Japanese Patent Application Laid-Open (JP-A) No. 58-127912 discloses a technique in which optical scanning ranges of a pair of light beams for exposing adjacent partial exposure regions are overlapped at the joint of the partial exposure ranges (the end portions of the optical scanning ranges of both light beams are set to be overlapped along a direction perpendicular to the scanning direction). The position of the joint of the partial exposure ranges formed by both light beams is set randomly within a region (hereinafter called a boundary region) in which the optical scanning ranges are overlapped. Thus, no striped pattern generated at the joint of the partial exposure ranges is apparently conspicuous.
In Japanese Patent Application Laid-Open (JP-A) No. 3-98066, by decreasing the exposure amount of one light beam and increasing the exposure amount of the other light beam in the same ratio in a boundary region which is formed by the pair of light beams and in which the light scanning ranges are overlapped in the boundary region, a total of exposure amounts of the pair of light beams in the boundary region is set to an average value not greatly different from an exposure amount in another exposure range (an exposure range which is other than the boundary region and which is exposed by a single light beam).
In the technique described in Japanese Patent Application Laid-Open (JP-A) No. 58-127912, the position of the joint of the partial exposure ranges formed by the light beams in the boundary region is set randomly so that the position of an end portion of a range actually exposed by each of the light beams varies randomly. Accordingly, when an image is to be formed on the photosensitive body, image data expressing the image to be formed must be divided into plural data in accordance with the position of the joint which is varying randomly, and each of the light beams must be modulated at a timing corresponding to the randomly-varying position of the joint by using the divided data. Accordingly, a problem exists in that processing becomes very complicated. Further, when the position of the joint of the partial exposure ranges is periodically varied in order to simplify the processing, the image is periodically disturbed.
In the technique described in Japanese Patent Application Laid-Open (JP-A) No. 3-98066, the exposure amount in the boundary region can be averaged. However, the irradiating positions of the pair of light beams in the boundary region, in which the optical scanning ranges are overlapped, are slightly offset from each other, and thus, the image is blurry in the boundary region exposed by each of the pair of light beams. This bluriness is clearly visually recognized in particular when the resolution of the image to be formed is high. Accordingly, it is not preferable to apply this technique when an image is to be formed with high quality.
In each of the techniques described in the above publications, the offset in the irradiating positions of the light beams at the joint of the partial exposure ranges is made inconspicuous by varying the position of the joint or controlling the light amount. However, as mentioned above, problems exist in that processing becomes complicated and the image is disturbed or becomes blurry. Therefore, a technique for overcoming offset itself in the irradiating positions of the light beams at the joint of the partial exposure ranges is eagerly desired.
In connection with the above techniques, in Japanese Patent Application Laid-Open (JP-A) No. 1-183676, in an image forming device for forming a color image or the like by using plural light scanning devices, amounts of offset of the formed positions of the images of respective colors formed by the respective light scanning devices are detected by a CCD sensor. The scanning lines of the images of the respective colors, which are overlapped as a color image, are registered by adjusting an optical system or a synchronous system on the basis of the results of detection of the amounts of offset.
However, in the technique described in this publication, the position detection is performed by an expensive CCD sensor or the like to register the positions of the scanning lines. Further, the structure of the image forming device becomes complicated in order to control operations of the optical system and the synchronous system. Moreover, since it is necessary to perform complicated control, it takes time to perform processing. In addition, in the above publication, only registration in an image unit is disclosed, and there is no disclosure of how to register the irradiating positions of light beams at a joint of partial exposure ranges when the exposure range is divided into plural partial exposure ranges and each of the partial exposure ranges is exposed by a separate light beam.